Filters containing discrete media are widely employed for the removal of suspended and colloidal impurities, such as organic matter, plankton, sewage, vegetable matter, colloidal silica, oil, and corrosion products from liquids, such as water. In some cases, coagulants are added to the liquid stream before filtration. The filters may include conventional single-material beds or multiple-material beds composed of sand, coal, or the like. The media may be, for example, spherical, or cylindrical.
During a filtration operation, the liquid to be filtered moves through the media under the influence of gravity, or, alternatively, the media may be enclosed in a pressure vessel and the liquid to be filtered is positively forced through the bed. Suspended matter is strained from the liquid stream and accumulates in voids between the solid particles of media producing a progressive increase in pressure across the bed. When the pressure drop reaches a predetermined limit, the filter is backwashed to discharge the accumulated material.
The media is backwashed by moving a liquid material, such as water, upward through the bed at rates that cause the filter bed to expand but not to fluidize completely. Additionally, a compressed gas, such as air, is often introduced into the bottom of the media bed to further agitate the bed and to break up crust and mud balls lodged in the bed. If introduced simultaneously with a backwashing liquid, the compressed air forms bubbles which rise through the expanded bed further increasing turbulence.
Turbulence caused by the upward velocity of the backwash liquid and by the motion of gas bubbles in the bed is desirable for dislodging dirt from the media, but tends to carry particles of the media up and into a collection outlet or trough where the particles are carried away and lost from the filter. Accordingly, relatively large freeboard water spaces are commonly provided over the volume of the expanded bed during backwashing to promote separation of filter media particulate from the dirt and the backwashing liquid.
In addition, baffles have been proposed, as described in U.S. Pat. No. 2,453,345 issued to Row et al. The baffles are said to reduce turbulence in the vicinity of a collection outlet, so that the media separates from the backwash stream and returns to the filter bed. The patent purports to teach a baffle system including a vertical baffle on each side of a collection trough. At lower corners of the collection trough, two more baffles are mounted which extend outwardly and downwardly to define gaps between the different types of baffles. However, separation efficiency of the system is reportedly hampered by eddy currents created by the interaction of the baffle system with rising air bubbles.
Another baffle system is described in U.S. Pat. No. 4,076,625 which issued to Scholten et al. The system includes one or more arcuate baffles positioned near a collection trough. The arcuate baffles purportedly divert unwanted eddy currents away from the collection trough and away from filter media which is being returned to the filter bed. Unfortunately, the purported teachings described by Scholten et al. appear to be limited to configurations which include but a single outflow channel. The system purportedly taught by Scholten et al. requires each of the returning media particles to pass countercurrently through the entire stream of rising backwash liquid.
Accordingly, there still exists a need for an improved baffle system. Preferably, such an improved baffle system would minimize the effects of unwanted eddy currents, provide for the unhindered return of media to the filter bed, and be of simple and straightforward construction that is adaptable to all types of discrete media filters.